About 2.4 billion years ago on Earth, something known as the Great Oxygenation Event occurred when Earth's lower atmosphere began to become rich in O2 as oxygen sinks such as dissolved iron and organic material became saturated and couldn't hold any more. Before that, the air near the surface of our planet contained less than 0.001 percent of today's oxygen level. While we've known this for a while, scientists were never quite sure what the Earth's upper atmosphere looked like at the time. Now, by examining fossilized cosmic dust, a team of researchers has sorted it out.

The team, led by Andrew Tomkins from Monash University, extracted tiny cosmic dust particles – also technically called micrometeorites – from limestone where they had been fossilized. The limestone came from the Pilbara region in Western Australia. According to a statement from Monash, these are the oldest fossilized micrometeorites ever found, dating to about 2.7 billion years ago.

These particles were examined with cutting-edge microscopes which revealed that while they had once been iron, they were converted to iron oxide in the upper atmosphere. Based on just how the micrometeorites were converted, the scientists determined that the levels of oxygen needed to cause the reaction in the upper atmosphere had to have been close to today's values.

"This was an exciting result because it is the first time anyone has found a way to sample the chemistry of the ancient Earth's upper atmosphere," Tomkins says.

So if the Earth's surface was oxygen poor 2.7 billion years ago, but its upper atmosphere was oxygen rich, what exactly was going on?

The researchers believe that there might have been striations in the atmosphere, with a layer of methane separating the lower from the upper atmospheres. Such a layer would absorb ultraviolet light causing a kind of heat shield that would prevent the layers from mixing. The carbon dioxide on the outside of this layer would get beamed by even more UV light, causing it to breakdown and release oxygen, which accumulated in the upper atmosphere.

"It is incredible to think that by studying fossilized particles of space dust the width of a human hair, we can gain new insights into the chemical makeup of Earth's upper atmosphere, billions of years ago," Tomkins says.

According to Tompkins, the next phase of the research will focus on extracting and examining even more micrometeorites to glean additional insights about the Earth's early atmospheric chemistry, in particular the time around the GOE, which is also known as the Oxygen Catastrophe, Oxygen Crisis and Oxygen Holocaust, as it caused the death of much of the world's anaerobic organisms at the time.

The team's findings were reported on May 11 in the journal Nature.

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